The present invention relates to a method for fabricating a thyristor.
A high-voltage direct-current-transmission system must be able to handle a voltage of several kilovolts to several hundred kilovolts. Such a system frequently comprises one hundred or more series-connected thyristors. A conventional single thyristor which is generally used has an off-state voltage (blocking voltage) of only about several kilovolts. When a plurality of such thyristors are used in a single system, the system is complicated in configuration. In order to provide a compact system having a small number of thyristors and improve the reliability thereof, an off-state voltage of a thyristor must be increased.
However, according to a conventional fabrication method, when an off-state voltage of a thyristor is increased above a certain limit, three important characteristics (i.e., on-state voltage (VF), turn-off time (tq) and reverse recovery charge (Qrr)) of a thyristor are also increased, thereby increasing variations in respective characteristics. In order to prevent this, the system becomes undesirably large.
The above problem will be described in detail hereafter. A conventional thyristor is fabricated in the following manner. P-type impurities are diffused in two surfaces of an n-type Si substrate to obtain a pnp wafer having a p-emitter and p- and n-bases. A portion of a surface of the p-base is etched to control the thickness thereof. Thereafter, oxide films are formed on two surfaces of the pnp wafer. A portion of one oxide film is removed by a known photoetching process. n-type impurities are selectively diffused in the p-base to form an n.sup.+ -emitter. In this process, phosphorus (P) is deposited to several micrometers, and is then driven in at a high temperature. Then, the n.sup.+ -emitter has a depth of about 20 .mu.m. Thereafter, p-type impurities are diffused in two surfaces of the obtained pnpn wafer to have a high surface impurity concentration but a lower concentration than that of the n.sup.+ -emitter, thereby forming a p.sup.+ -layer. A heavy metal such as gold (Au) is diffused in the pnpn wafer obtained as described above. Finally, predetermined electrodes are formed on the resultant structure.
In order to increase an off-state voltage of the thyristor formed in the above manner, that is, to have a high breakdown voltage, a thickness and a resistivity of the n-base must be increased. On the other hand, a thyristor used in a direct current transmission system described above must have an on-state voltage (VF) of about 3 V or less at a current density (J) of about 100 to 150 A.cm.sup.-2, a turn-off time (tq) of 500 .mu.sec or less, and variations in a reverse recovery charge (Qrr) of .+-.20% or less. In a thyristor according to a conventional fabrication method, a maximum off-state voltage is 4 kV which satisfies the above conditions. A thyristor having an off-state voltage of more than 4 kV cannot be practically obtained.
In order to solve the above problem, the following methods have been suggested.
According to one of these methods, as described in Japanese Patent Publication No. 57-26424, gettering is performed on a pnp wafer. Thereafter, phosphorus is diffused at a low temperature to form an n.sup.+ -emitter. According to another method, as described in U.S. Pat. No. 4,370,180, after forming an n.sup.+ -emitter, gettering is performed on a pnpn wafer immediately before a gold diffusion process. However, according to these methods, a thyristor which satisfies the above conditions and has an off-state voltage of more than 6 kV cannot be practically obtained.
The reason for this is as follows. In the former method, the n.sup.+ -emitter is formed at a low temperature after the gettering process, thereby undesirably prolonging a heating process. In this n.sup.+ -emitter forming process, a plurality of defects are introduced. These defects getter a heavy metal. Therefore, a carrier lifetime of p- and n-bases in the vicinity of the n.sup.+ -emitter is undesirably prolonged, thereby excessively prolonging a turn-off time of a thyristor. In the latter method, the gettering is performed after forming the n.sup.+ -emitter. A plurality of defects are introduced in the same manner as the former method. In this method, a carrier lifetime is finally controlled by gold diffusion. However, since a plurality of defects trap the gold, a carrier lifetime distribution cannot be precisely controlled. Therefore, according to this method, a thyristor which has an off-state voltage of more than 6 kV and a sufficiently small on-state voltage (VF) and turn-off time (tq) cannot be obtained.
In any such method, when the n.sup.+ -emitter if formed, impurities are diffused to have a high concentration. As a result, a plurality of defects are formed, thereby degrading the characteristics of a thyristor.